In an aircraft gas turbine (et) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.
The compressor and the bypass fan are both rotating structures in which stages of blades extend radially outwardly from a respective compressor or bypass fan rotor disk. The compressor blades have complexly shaped and curved airfoils that compress the air to progressively higher pressures for injection into the combustors. The fan blades are also complexly shaped and curved to force the air around the center core of the engine and out the trailing end of the engine. The compressor rotor disk and the bypass fan rotor disk turn at thousands of revolutions per minute. In a large gas turbine engine the compressor blades and bypass fan blades may be quite long and extend a substantial distance from the centerline of the engine. Consequently, both the compressor blades and bypass fan blades move through the air at a high velocity.
The compressor blades and the bypass fan blades receive the inward flow of air into the gas turbine engine at a combined velocity determined both by their rotational velocity and by the relative velocity of the engine through the air. The combined velocity is typically at least near Mach 1, and may be considerably greater than Mach 1 in many situations. Any solid or liquid particles in the air—dust, dirt particles, sand, fine water droplets, raindrops, ice, and snow, for example—impact against the compressor blades and the bypass fan blades at the combined velocity. These particles may be of a wide range of masses, from lightweight particles to relatively heavy particles, but are not so heavy that they cause instantaneous fracture of the blades (as could be the case for an ingested bird or the like). Because of the complex shapes of the airfoils of the compressor blades and the bypass fan blades and the change in the combined velocity under different flight conditions, the solid particles impact the various regions, and even the same region, of the blades over a variety of particle-impact angles of incidence.
The particle impacts may collectively cause substantial amounts of particle-impact damage to the compressor-blade airfoils and to the bypass-fan-blade airfoils. In some cases, no action is taken to avoid this damage, which in turn leads to earlier repair or replacement of the compressor blades and/or the bypass fan blades than would otherwise be necessary. In other cases, there have been attempts to apply protective coatings to the surfaces that are impacted by the particles. The most commonly used of such protective coatings is tungsten carbide-cobalt material having particles of tungsten carbide dispersed in a cobalt matrix. The coating material is very heavy and adds to the rotating weight of the compressor blades and bypass fan blades, which in turn leads to greater shaft, bearing, and structural weights. Such coatings are also subject to spallation during service.
There is accordingly a need for an improved approach to the protection of gas turbine components, such as compressor blades and bypass fan blades, and other articles as well, against the damage caused by high-velocity particle-impact damage. The present invention fulfills this need, and further provides related advantages.